The present invention relates in general to analog to digital converters and in particular to computer based analog to digital converters of the type employing iterative processes to achieve high resolution conversion.
Analog to digital (A/D) converters typically utilize costly precision resistors and as the resolution and accuracy of a converter are increased, so too are the number of precision resistors in the converter. For instance, a ladder network A/D converter of the prior art utilizes a precision resistor network to produce a plurality of reference voltages of differing magnitudes. An input voltage is compared with each reference voltage and the approximate magnitude of the input voltage is found by determining which reference voltages are larger and which reference voltages are smaller than the input voltage. The number of precision resistors and comparators required increases with the resolution of the converter, and therefore high resolution A/D converters of this type are costly.
A typical ramp type A/D converter of the prior art utilizes a D/A converter to produce a reference voltage which is compared by a comparator to an input voltage to be digitized. The reference voltage is incrementally increased by increasing input data to the D/A converter until the comparator indicates that the reference voltage exceeds the input voltage. At this point the input data driving the D/A converter approximates the input voltage. Although the D/A converter utilized in a ramp A/D converter usually contains precision resistors and a comparator, fewer precision resistors and comparators are required by a ramp A/D converter than is required by a ladder network A/D converter for the same degree of resolution. Nonetheless, the number of precision resistors required increases in proportion to the desired resolution so that high resolution ramp type converters are also expensive.
Some computer-based A/D converters use iterative processes to expand the resolution of an A/D converter. One such computer-based A/D converter, described in U.S. Pat. No. 4,555,692, issued Nov. 26, 1985 to Eng, Jr. et al, is a "recirculating remainder" A/D converter employing a conventional ramp type A/D converter which initially digitizes an input voltage to produce a low resolution "first pass" digital representation of the input voltage. The difference between the input voltage and the final reference voltage produced by the ramp A/D converter during the first pass measurement (the "remainder voltage") is then amplified and applied as a "second pass" input to the ramp A/D converter which then produces a digital representation of the remainder voltage. The second pass converter output data is then divided and added to the first pass converter output data to produce a higher resolution "second pass" representation of the input voltage. A second remainder voltage comprising the amplified difference between the first remainder voltage and the final reference voltage produced by the ramp A/D converter during the second pass is then digitized by the ramp A/D converter during a "third pass" through the converter with the result being divided and added to the second pass representation to produce a still higher resolution "third pass" representation of the input voltage. The process may be repeated several times to obtain progressively higher resolution representations of the input voltage. However noise introduced by the measurement apparatus usually becomes the predominant source of the remainder voltage after not more than five passes and the accuracy of the output of such recirculating remainder A/D converters is typically limited to about 18 significant bits.
In addition to digitizing input voltages with high resolution and accuracy, a precision A/D converter should also operate with a high degree of linearity over a wide input range, should be easy to calibrate, and should remain calibrated for long periods. Precision A/D converters of the prior art typically require use of specialized test equipment and careful manual adjustment of variable capacitors or potentiometers for calibration.